In this disclosure, the term “wireless terminal” is used to represent any communication entity capable of radio communication with a wireless network by sending and receiving radio signals. For example, the wireless terminal described herein may be any of a mobile telephone, a tablet, a laptop computer and a Machine-to-Machine, M2M, device, also known as Machine Type Communication, MTC, device. Another common generic term in this field is “User Equipment, UE” which may be used herein as a synonym for wireless terminal.
Further, the term “network node”, is used herein to represent any node of a wireless network, which node is operative to transmit radio signals in multiple spatially differentiated beams, known as beamforming, in radio communication with wireless terminals. The network node in this disclosure may refer to a base station, radio node, Node B, base transceiver station, access point, etc., which communicates radio signals with the wireless terminal.
Beamforming is likely to become an important technology in future radio communication systems and wireless networks. It can be used to improve performance in a wireless network, by increasing the received signal strength, thereby improving the radio coverage in the network, and also by reducing unwanted interference, thereby improving the capacity of the network. Beamforming can be applied both in a signal transmitting node, referred to as a transmitter, and a signal receiving node, referred to as a receiver. In a transmitter, beamforming amounts to configuring the transmitter to transmit the signal in a specific direction (or a few directions), referred to as a transmit beam, and not in other directions. In a receiver, beamforming amounts to configuring the receiver to only receive signals from a certain direction (or a few directions), and not from other directions, referred to as a receive beam. When beamforming is applied in both the transmitter and the receiver for a given communication link, the term beam-pair link (BPL) is commonly used for the combination of beams selected in the both ends, i.e. including the above-mentioned transmit beam and receive beam. A BPL can also be referred to as a beam-tracking process (BTP) since it may be subject to different procedures for its maintenance.
In a radio communication of signals transmitted from a network node and received by a wireless terminal, a selected network beam and a selected terminal beam are used. Throughout this disclosure, the term “selected” implies that the beams are used for communicating data and/or messages between network node and the wireless terminal. During the communication, the available network beams and terminal beams are evaluated based on signal measurements and if there is another combination of network beam and terminal beam which is better than the currently used combination, the new combination is used instead. The above evaluation can be performed by the wireless terminal when the network node transmits certain reference signals on different network beams, by measuring received power of a beam reference signal using different candidate terminal beams. This process is referred to as beam tracking.
However, the above-described beam tracking and signal measurements is a slow process and in the meantime the measurements may become outdated so that the selection of beam(s) based on those measurements may not be optimal.